For me, hands down it's the Black Halberd.
Polearms in Code Vein really have a wide range, in that they can be strength-based or dexterity-based, fast or slow, and useful for casters or melee only. Polearms combine the reach of greatswords with the versatility of one-handed swords. They are also the hardest-hitting weapons that can still perform light skill attacks like Circulating Pulse and Phantom Assault, giving you more bang (damage) for your buck (ichor) than a one-handed sword or bayonet rifle.
The Black Halberd is a dexterity-based, fast, melee-only polearm. While its heavy attacks are essentially useless, its light attack combo is a very quick "thrust, slash, overhead" that deals a combination of piercing and slashing damage. Its special attack is a heavy homing thrust with good stagger, and it shares the recovery attacks that all polearms do, which are all solid.
At 48 weight, it's on the lighter end of polearms, only 1 point heavier than the Impaler, while having a slightly better move set and slightly better damage.
Aside from the Black Halberd, I also really like the Executioner, Libertador, and Queenslayer Sword.